Combustion control circuit for a fuel burner

ABSTRACT

A combustion control circuit for an oil burner installation includes a triac for controlling the energization of both a burner motor and an ignition circuit. A control circuit carries out a safe start component check and then operates the triac when a thermostat closes and when no flame is detected at the burner. If ignition fails to take place in a trial-for-ignition period of predetermined time, a thermal switch is heated by a heater in the control circuit and opens to shut off the burner motor. However, if ignition takes place, flame is detected within the predetermined period by an improved, positively acting, reliable resistance bridge flame detector circuit. The flame detector circuit serves to render nonconductive a silicon controlled rectifier in the control circuit in order to disconnect the thermal switch heater while maintaining operation of the burner motor. Simultaneously, the ignition circuit is deenergized by a novel ignition disabling circuit connected between the silicon controlled rectifier and the ignition circuit. Failure of the triac in a short circuit condition prevents an unsafe condition due to connection of the thermal switch to the motor and due to the use of a second heater in circuit with the ignition circuit.

United States Patent 1 Lourigan 1 May 8, 1973 [54] COMBUSTION CONTROLCIRCUIT [57] ABSTRACT FOR A FUEL BURNER A combustion control circuit foran oil burner installa- [75] Inventor: Ronald F. Lourigan, Kenosha, Wis.tion includes a triac for controlling the energization of AssigneezWebster Electric Company, Inc. both a burner motor and an ignitioncircuit. A cintrol Racine wiscircuit carries out a safe start componentchec and then operates the triac when a thermostat closes and [22]Filed: May 25, 1972 when no flame is detected at the burner. If ignitionfails to take place in a trial-for-ignition period of [21] Appl' 256381predetermined time, a thermal switch is heated by a heater in thecontrol circuit and opens to shut off the [52] U.S. Cl. ..307/117,431/69, 431/74 burner motor, However, if ignition takes place, flameInt. CL. is detected within the predetermined period an im of Search l6,1 17, 1 18, proved positively acting reliable resistance 307/119, 112;431/24, 25, 2 69, flame detector circuit. The flame detector circuit 27serves to render nonconductive a silicon controlled rectifier in thecontrol circuit in order to disconnect References Cited the thermalswitch heater while maintaining operation UNITED STATES PATENTS of theburner motor. Simultaneously, the ignition circult is deenergized by anovel ignition disabling circuit 3,463,600 8/1969 Axmark ..307/1 17 Xconnected between the silicon controlled rectifier and 3.584.988 6/1971Hirsbflmner et the ignition circuit. Failure of the triac in a shortcir- 3,624,407 ll/l97l Bauer ..307/1 16 Quit condition prevents anunsafe condition due to Primary ExaminerRobert K. Schaefer AssistantExaminerM. Ginsburg Attorney-Richard D. Mason et al.

connection of the thermal switch to the motor and due to the use of asecond heater in circuit with the ignition circuit.

26 Claims, 3 Drawing Figures COMBUSTION CONTROL CIRCUIT FOR A FUELBURNER The present invention relates to an improved combustion controlcircuit for reliably and safely controlling the operation of a fuelburner.

A conventional fuel oil burner heating installation of a type widelyused in the past includes an ignition device such as a transformer orother ignitor for igniting fuel at the burner, together with a primarycontrol using two or more relays for operating the burner in response toa thermostat. Upon closing of the thermostat in normal operation, theburner motor and the ignition device are energized during atrial-for-ignition period. If burner flame is detected, the burner motoris continued in operation and, if the installation is of theintermittent ignition type, the ignition device is deenergized. n theother hand, if flame is not detected during the initial period, athermal delay safety switch shuts down the installation.

The complexity and expense of conventional control systems has made itdesirable to provide a safe, economical and reliable combustion controlcircuit making use of inexpensive solid state components in place ofexpensive relays, large transformer windings, and the like. Devices ofthis general class have been proposed, and one example may be found inU. S. Pat. No. 3,624,407 Bauer. However, solid state devices developedto date have suffered from several disadvantages such as lack ofcomplete safety in operation, unreliability under somecircumstances,'the need for expensive components, and excessive costand/or complexity.

Objects of the present invention are to provide an improved combustioncontrol circuit for controlling the operation of a fuel burnerinstallation including a burner motor and an ignition circuit; toprovide an improved combustion control circuit having desirable safetyfeatures including a novel arrangement for protecting against componentfailure; to provide an improved combustion control circuit in which thenecessity for expensive components is reduced; to provide an improved,highly reliable flame detection circuit for use in a combustion controlcircuit; and to provide a combustion control circuit having an improvedarrangement for discontinuing the operation of the ignition circuit whenburner ignition is accomplished.

in brief, the above and other objects and advantages of the inventionare realized by the provision of an improved combustion control circuitfor an oil burner installation of the type including a burner motor andan ignition circuit. A triac switching device is connected forsimultaneously energizing both the burner motor and the ignitioncircuit. A thermostat is located in the area to be heated by the oilburner installation, and a light sensitive variable resistance flamedetector is disposed adjacent the fuel burner. A control circuitoperates to place the triac in a conductive condition to energize boththe burner motor and the ignition device when the thermostat closes upondemand for heat. When ignition is accomplished, the control circuitoperates in response to the flame detector for discon tinuing operationof the ignition circuit. On the other hand, if ignition is notaccomplished during a predetermined period of time, a thermal delayswitch connected to the burner motor prevents further operation of theburner motor upon being heated to a predetermined extent. A first heaterin the control circuit operates from the time that the thermostat closesuntil such time as a flame is detected. A second heater for the thermalswitch protects against failure of the triac and heats the thermalswitch at all times that the ignition circuit is in operation.

In accordance with another feature of the invention, there is provided anovel flame detection circuit comprising a resistance bridge providedwith a regulated operating potential and including a light sensitivevariable resistance flame detector in one leg thereof. In the absence offlame the resistance bridge is maintained in an unbalanced condition.Upon the detection of flame, the resistance bridge is moved through thenull point to an oppositely unbalanced condition, and the controlcircuit is operated positively and effectively.

An aspect of the invention resides in a novel circuit for disabling theignition circuit in response to the detection of flame. In oneembodiment, a transformer coupled between a switching device in thecontrol circuit and a controlled conduction device in the ignitioncircuit enables or prevents operation of the ignition circuit inresponse to presence or absence of flame. In other embodiments, a relayand a light and light sensitive device are used.

The invention and its objects and advantages may be best understood fromconsideration of the following detailed description of embodiments ofthe invention shown in the accompanying drawing, wherein:

FIG. 1 is a schematic and diagrammatic illustration of a combustioncontrol circuit embodying the principles of the present invention; and

FIGS. 2 and 3 are fragmentary schematic illustrations of alternativeembodiments of the invention.

With reference now to FIG. I of the drawing, there is illustrated acombustion control circuit designated generally by the reference numeral10 and constructed in accordance with the principles of the presentinvention. The circuit 10 controls the operation of a typical oil burnerinstallation including a burner 12, a burner motor 14 and an ignitioncircuit generally designated by the reference numeral 16. In normaloperation, the burner motor 14 and the ignition circuit 16 are operatedupon a demand for heat by a burner circuit generally designated as 18under the control of a control circuit generally designated as 20.Demand for heat is signalled in conventional manner by a line voltagethermostat switch 22 or by a low voltage thermostat switch 24 located tosense temperature in a region heated by operation of the burner 12. Inany given installation only one of the switches 22 or 24 is used, theother being replaced by a permanent closed circuit.

Upon closing of the controlling thermostat switch 22 or 24, fuel at theburner 12 is ignited by the ignition circuit 16 and flame is detected bya flame detection circuit generally designated as 26. Upon detection offlame, the control circuit 20 serves to continue operation of the burnermotor 14 while an ignition disabling circuit generally designated as 28discontinues further operation of the ignition circuit 16. Shouldignition-fail to take place within a predetermined time, a thermal delaydevice designated as a whole by the reference numeral 30 discontinuesoperation of the fuel burner installation. In accordance with a featureof the invention, the device 30 also discontinues operation of theinstallation in the event of certain undesirable malfunctions ofcomponents in the circuit 10.

Referring now in more detail to the construction and operation of thecombustion control circuit 10, the circuit includes a pair of powersupply terminals 32 and 34 adapted to be connected to a standard nominal115 volt, 60 hertz power supply. The control circuit 20 and the flamedetection circuit 26 are supplied with operating potential by means of apair of secondary windings 36 and 38 of a transformer 40 having aprimary winding 42 adapted to be coupled to the power supply terminals32 and 34. In addition to the line voltage thermostat switch 22 (whenused), energization of the burner circuit l8 and the control circuit 20can be interrupted by a conventional over-temperature limit switch andfan switch 44 and by operation of the thermal switch structure 30.

In operation of the circuit 10, control of the energization of theburner circuit 18 is provided by a gated, bidirectional solid statecontrolled conduction device 46 in the form of a triac. The outputelectrodes of triac 46 are in current supplying relation with both theburner motor 14 and with the ignition circuit 16. Thus triac 46 must berendered conductive by coupling of its gate electrode to one of itsoutput electrodes in order to bring about operation of the burnercircuit 18.

In FIG. 1 the combustion control circuit is illustrated in its normal,temperature satisfied condition wherein the thermostat switch 22 or 24is open and wherein the triac 46 is in its high resistance or opencircuit condition so that neither the burner motor 14 nor the ignitioncircuit 16 is energized. When a demand for heat is sensed by closing ofthe controlling thermostat switch 22 or 24, a heating cycle is initiatedunder the control of the control circuit 20. Upon initiation of thisheating cycle, a safe start component check takes place, and assumingthe circuit components to be operating properly, the control circuitserves to render the triac 46 conductive thereby simultaneouslyenergizing the burner motor 14 and the ignition circuit 16 throughout atrial-for-ignition period having a duration determined by the operationof the thermal device 30.

In order to operate the triac 46, control circuit 20 includes a pair ofrelay windings 48 and 50, which windings control a normally open set ofrelay contacts 52 connected in series with a resistor 54 and with thegate electrode of the triac 46. Windings 48 and are arranged in knownmanner relative to contacts 52 through adjustment of variableresistances 49 and 51 respectively so-that energization of both windingsis necessary to move or pull in the set of contacts from the open to theclosed position, while energization of only the winding 50 suffices tohold the contacts in the tant features of the invention, the circuit 26carries out a stable and reliable control operation despite suchunfavorable factors as supply voltage fluctuations, intermittent orunstable ignition, intermittent extraneous light, or the like.

More specifically, circuit 26 includes a zener diode 58 serving togetherwith a resistor 60 to supply a voltage regulated driving signal to aresistance bridge 62.

Due to the inclusion of diode 58, the bridge 62 operates substantiallyindependently of supply voltage variations. Bridge 62 includes a pair ofinput terminals 64 and 66 connected across the zener diode 58 and a pairof output terminals 68 and 70 connected to the gate and cathodeelectrodes of the SCR 56 and shunted by a capacitor 72.

One leg of bridge 62 includes a variable resistance photoelectric device74 disposed adjacent the burner 12. In the absence of flame the device74 is in a high resistance condition, and when illuminated by thepresence of flame the device 74 assumes a low resistance condition. Theremaining three legs of bridge 62 include resistances 76, 78 and 80having values such that the bridge 62 is unbalanced in oppositedirections when the device 74 is alternatively in its high and lowresistance conditions.

The novel flame detection circuit 26 has important advantages inoperation because it reliably and positively controls the condition ofthe SCR 56. When no flame is present at burner 12, device 74 is in itshigh resistance condition and as a result the bridge 62 is unbalancedand bridge terminal 70 is maintained at a positive potential relative tobridge terminal 68. This potential appears across capacitor 72 in such away as to maintain the gate electrode of the SCR 56 positive relative toits cathode. Accordingly, the SCR is maintained in condition forconduction of current through its output circuit. When flame is detectedthe resistance of device 74 drops to a low level and the bridge 62 movesthrough its null point or balance condition to an oppositely unbalancedcondition wherein terminal 70 is negative relative to terminal 68, thusbiasing the gate electrode of the SCR 56 negative relative to itscathode to assure that SCR 56 ceases conduction and remainsnonconducting.

Capacitor 72 provides asmoothing or filtering effect which preventsunwanted response of the flame detection circuit to various possibletransient effects. Moreover, since the incident light level at device 74must change substantially enough to change the condition of the bridgefrom one unbalanced condition to the oppositely unbalanced condition,positive and reliable operation is achieved. Furthermore, stability ofoperation is aided by the use of the zener diode 58 which provides aregulated driving voltage for the bridge 62 substantially independent ofvariations in the supply voltage.

As indicated above, upon a demand for heat the operation of thecombustion control circuit 10 commences with a safe start componentcheck. When switch 22 or 24 closes, a circuit is completed forenergization of the control circuit 20 from the transformer 40. At thistime the triac 46 is rendered conductive only if the components of theflame detection circuit 26 and control circuit 20 are operational. Ifthe photoelectric device 74 provides a false indication of flame, the

SCR 56 is nonconductive and winding 48 cannot be energized to closenormally open relay contacts 52. Similarly if SCR 56 is inoperative andcannot conduct current, winding 48 cannot be energized. The same resultfollows if because of any other fault the control circuit 20 or flamedetector circuit 26 fails to operate.

If all components are operational, closing of the thermostat switchcauses both windings 48 and 50 to be energized. More specifically,winding 50 is energized at a continuous DC level by a diode 82 andcapacitor 84 connected across secondary winding 38 in parallel with aresistor 86. Winding 48 is energized simultaneously by current flowingthrough the conductive SCR 56 and through a resistor 87 associated withthe thermal delay circuit 30.

Simultaneous energization of windings 48 and 50 causes contacts 52 tomove to their closed condition thereby gating the triac 46 to itsconductive condition. Voltage fluctuations due to the inductive natureof the load and switching transients and the like are shunted by acircuit including a capacitor 88 and resistor 90 providing reliableturn-off of the triac 46. A further aspect of the safe start componentcheck is that the triac 46 cannot be rendered conductive following afailure of the relay which prevents contacts 52 from closing, or if thetriac itself fails in the open circuit condition.

Operation of both the burner motor 14 and the ignition circuit 16 takesplace when the triac 46 is rendered conductive. When the burner motoroperates, a combustible mixture of fuel and air is emitted at theburner, and is ignited by the ignition circuit 16 to produce a stableburner flame.

Referring more specifically to the structure of the ignition circuit 16,this circuit may take any desired form, and as illustrated includes apair of spark electrodes 92 and 94 located in ignition relation to thecombustible air fuel mixture at the burner 12. Although other typescould be used, the ignition circuit 16 is similar to the ignitioncircuit illustrated and described in detail in U. S. Pat. No. 3,556,706Campbell, to which reference may be had for a more complete disclosure.Briefly, the circuit 16 includes a capacitor 96 which chargesrepetitively during alternate half-cycles of the power supply waveformthrough a diode rectifier 98, a resistance 100 associated with thethermal delay circuit 30, and an inductance 102. Repeatedly during thehalfcycle, as the voltage across the capacitor 96 reaches apredetermined threshold level, a trigger circuit including a pair ofvoltage divider resistors 104 and 106 and a phase shift capacitor 108applies an operating voltage to the gate electrode of an ignitioncircuit SCR 110. At this point the SCR 110 is placed in a conductivecondition and rapidly discharges the capacitor 96 in a pulse or surgethrough the primary winding 112 of a transformer 114 having a secondarywinding 116 connected to the spark gap electrodes 92 and 94. As aresult, during alternate half-cycles of the power supply waveform thereare produced adjacent the burner 12 a high frequency series of discrete,high energy ignition sparks.

Following initial energization of the burner motor 14 and ignitioncircuit 16, the combustion control circuit carries out atrial-for-ignition operation during a predetermined time period theduration of which is established by operation of the thermal delaydevice 30. In normal operation, ignition takes place during this period.If ignition fails to occur, then continued emission of fuel from theburner 12 could create an undesirable and potentially unsafe condition.For this reason, the thermal delay device functions to shut down theburner motor 14 and the combustion control circuit 10 at the end of thetrail-for-ignition period if ig' nition is not detected by the flamedetecting circuit 26.

Proceeding now to a description of the thermal delay device 30, thisdevice includes a normally closed set of switch contacts 118 controllingcurrent flowing via the triac 46 to the burner motor 14 and ignitioncircuit 16. Contacts 118 are controlled by a bimetallic switch actuator120 which opens the contacts 1 18 when the actuator is heated to apredetermined extent. Preferably, the contacts are arranged so that theyare latched open and cannot reclose until released by a service man oroperator. Heating of the actuator 120 is carried out at a relativelyhigh rate of thermal transfer by the resistor 87 in series with therelay winding 48 and the control circuit SCR 56. Heating at a slowerrate is carried out by the resistor in series with the triac 46. Theheat transferred to the actuator by the resistor 87 is such thatcontacts 1 18 open in about 30 seconds.

Although the thermal delay device 30 may take various forms, one devicehighly suitable for the purpose is disclosed in U. S. Pat. applicationSer. No. 204,491, filed Dec. 3, 1971, of Donald F. Dalziel and CharlesH. Heide. Reference may be had to that application for a furtherdescription of the structure and operation of the device 30.

Throughout the trial-for-ignition period, the heater resistor 87 isenergized because it is in series with the relay winding 48. If ignitionfails to occur within the predetermined time, contacts 118 open anddeenergize the burner motor 14 and the ignition circuit 16. The burnerinstallation remains shut down until such time as the cause for ignitionfailure is corrected and the contacts 118 are reclosed.

In normal operation, ignition occurs before opening of the contacts 118and the control circuit 20 places the combustion control circuit 10 inan operating condition wherein the burner motor 14 is continuouslyenergized and the ignition circuit 16 is disabled. The latter functionis carried out by the novel ignition disabling circuit 28.

More specifically, upon ignition a flame at the burner 12 is detected bythe photoelectric flame detecting device 74. in the manner describedabove, this results in the SCR 56 being placed in a nonconductivecondition. Accordingly, the heater resistor 87 of the thermal delaydevice is deenergized and actuator 120 is no longer heated by thisresistor. Although relay winding 48 is deenergized at this time,contacts 52 remain closed due to the continuing energization of therelay winding 50. The burner motor 14 continues to operate because thetriac 46 is maintained in its conductive condition.

When the control circuit SCR 56 becomes nonconductive, the ignitioncircuit SCR is disabled by the circuit 28. Ignition disabling circuit 28includes a coupling transformer 122 having a primary winding 124connected to the anode and cathode of the SCR 56 and a secondary winding126 connected to the gate electrode and cathode of the SCR 110. When SCR56 is nonconductive, current flows through the primary winding 124 inhalf cycles when the ignition circuit 116 would otherwise operate, andthe gate electrode and cathode of the SCR 110 are reverse biased. Due tothis reverse bias the SCR 110 cannot be rendered conductive and nocapacitor discharge takes place to produce ignition sparks. Conversely,when SCR 56 is conductive, it acts as a low resistance shunt of primary124 to prevent reverse biasing of the ignition circuit SCR ll 10.

FIG. 2 illustrates an alternative ignition disabling circuit generallydesignated as 28A. In this arrangement a relay winding 128 is energizedby a DC potential produced by a diode rectifier 130 and capacitor 132 toclose a normally open set of relay contacts 134. Closing of contacts 134effectively connects the gate electrode of SCR 110 to its cathode andprevents firing of the SCR 110. When SCR 56 conducts, it provides a lowresistance bypass of the winding 128 and contacts 134 remain open,permitting SCR 110 to operate in its normal fashion in ignition circuit16.

FIG. 3 illustrates yet another alternative ignition disabling circuitgenerally designated as 288. In this arrangement a light sensitiveresistance 140 is coupled between the gate electrode and cathode of theignition circuit SCR 110. A lamp 142 is disposed to illuminate theresistance 140 and is connected across the output electrodes of the SCR56. When SCR 56 conducts, it shunts the lamp 142 so that resistance 140is-in its high resistance condition and the ignition circuit operates.When SCR 56 is biased to its nonconductive condition, the lamp 142 isenergized to reduce the resistance of the device 142 and disable theignition circuit.

If at any time during normal operation the burner flame should beextinguished, another trial-for-ignition period is commenced. If flamedetecting device 74 does not detect flame, the SCR 56 is operated onceagain. The ignition circuit 16 operates and the heater resistor 87 isenergized until such time as ignition is reestablished, or,alternatively, until the contacts 118 open.

Returning to a description of a heating cycle carried out by thecombustion control circuit 10, after ignition 4 the burner motor 14continues to operate and the flame at the burner 12 heats the regionincluding the thermostat 22 or 24. When the demand for heat issatisfied, the triac 46 is rendered nonconductive and the installationreturns to its initial temperature satisfied condition.

One important feature of the present invention is that the circuitincludes novel structure to prevent against unsafe conditions arisingfrom failure of the triac 46 in a short circuit condition wherein itremains conductive regardless of the control signal applied to its gateelectrode. This is a common failure condition of such devices. In priorart units if a controlling triac shorts out, the burner motor isenergized without any monitoring by the flame detector, and withoutcontrol of the thermostat in some cases, giving rise to a highlyundesirable and potentially unsafe condition. Thus, in prior circuits ithas been necessary to purchase expensive triac devices of a high qualityin order to reduce the probability of failure as much as possible.

In the circuit 10, failure of the triac 46 does not lead to an unsafecondition. Should such a failure occur with a control circuit thermostat24 in the open position, the

burner motor 14 and ignition circuit 16 are energized, and the controlcircuit 20 exercises no monitoring function. Moreover, the ignitioncircuit 16 is not disabled by the SCR 56 and the disabling circuit 28 or28A. However, the continuously operating ignitioncircuit l6 drawscurrent through the heater resistor of the thermal delay device 30.After a period of several minutes or so, the resistor 100 heats theactuator and the contacts 118 open to shut down the installation in asafe condition.

Should the triac 46 fail with a low voltage thermostat 24 in the closedcircuit condition, the circuit 10 operates normally until the thermostatopens. At this time the burner motor 14 continues to operate and theignition circuit 16 begins to operate. After a delay period, heaterresistor 100 heats the actuator 120 and the circuit 10 goes out onsafety.

When a line voltage thermostat is used, a potentially unsafe conditionexists in prior circuits wherein the thermal switch contacts are locatedin a low voltage circuit and not in series with the triac. In such anarrangement, should ignition fail to occur or should the flame beextinguished, the burner motor is maintained energized even if no flameis present because the thermal switch contacts are not in circuit withthe burner mo-.

tor. The present invention avoids this unsafe condition because if flameis not present and the thermal switch contacts open, current to theburner motor is positively discontinued without reliance upon theoperability of the triac 46.

Although the present invention has been described with reference to thedetails of the illustrated embodiments, it should be understood thatother modifications and embodiments will be apparent to those skilled inthe art. The details of the illustrated embodiments-are not intended tolimit the scope of the present invention as set forth in the followingclaims.

What is claimed and desired to be protected by Letters Patent of theUnited States is:

l. A combustion control circuit for controlling the operation of a fuelburner having a burner motor and an ignition device and comprising:

a triac in circuit with both the burner motor and the ignition device;

a thermostat;

a control circuit for placing the triac in a conductive conditiontoenergize both the burner motor and the ignition device in response tooperation of the thermostat;

a flame detector coupled to the control circuit for providing a flameindication in response to operation of the burner;

means coupled to said control circuit for disabling the ignition devicein response to a flame indication; I

a thermal delay switch for disabling the burner motor in response tobeing heated to a given extent;

a first heater connected in said control circuit for heating saidthermal delay switch during the time period beginning with operation ofthe thermostat and ending with production of a flame indication;

and a second heater connected in circuit with the ignition device forheating said thermal delay switch during operation of the ignitiondevice.

2. The combustion control circuit of claim 1, said thermal delay switchbeing connected in series with said triac.

3. A combustion control circuit for a fuel burner used to heat a spaceand including a burner motor and an ignition device, said combustioncontrol circuit comprising:

a switching means coupled to both the burner motor and the ignitiondevice and operable to energize the burner motor and ignition devicesimultaneously;

a flame detector disposed adjacent the fuel burner to provide a flameindication;

a thermostatically controlled switch in the heated space movable to anoperating position in response to a demand for heat;

a first control means coupled to said switching means and to saidthermostatically controlled switch for operating said switching means inresponse to movement of said thermostatically controlled switch to theoperating position;

a second control means coupled to said ignition device and to said flamedetector for discontinuing operation of said ignition device in responseto a flame indication;

a thermal delay switch connected to the burner motor for preventingoperation of the burner motor in response to being heated to apredetermined extent;

a first heater adjacent said thermal delay switch and connected to saidfirst and second control means for energization during operation of saidfirst control means prior to operation of said second control means; and

a second heater adjacent said thermal delay switch and connected to saidignition device for energization during operation of said ignitiondevice.

4. The combustion control circuit of. claim 3, said switching meanscomprising a triac.

5. The combustion control circuit of claim 4, said thermal delay switchbeing connected in series with said triac.

6. The combustion control circuit of claim 5, said first control meansincluding relay means having a winding means and having a contact meansconnected in controlling relation to said triac, and an SCR havingoutput electrodes and a gate electrode, said thermostatically controlledswitch being coupled to said output electrodes, and said flame detectorbeing connected to said gate electrode.

7. The combustion control circuit of claim 6, said flame detectorcomprising a light sensitive variable resistor, a bridge circuitincluding said detector in one leg, means for driving said bridge with aregulated driv ing signal, and a capacitor coupled to the bridge outputand to the gate electrode of the SCR.

8. The combustion control circuit of claim 5, said second control meanscomprising a coupling transformer connected between said flame detectorand said ignition device.

9. The combustion control circuit of claim 5, said second control meanscomprising a relay connected between said flame detector and saidignition device.

10. The combustion control circuit of claim 5, said second control meanscomprising a lamp and light sensitive resistor connected between saidflame detector and said ignition device.

11. A combustion control circuit for a fuel burner comprising:

control means for operating the burner in response to energization ofthe control means;

a controlled conduction device including electrodes forming an outputelectrode pair and a control electrode pair;

said output electrode pair being coupled to said control means forcontrolling the energization thereof;

a variable resistance flame detection device disposed in flame sensingrelation to the burner;

a resistance bridge having opposed input and output terminals and havingone leg including said flame detection device and having three legsincluding only fixed resistance; and

driving signal means coupled between a source of operating potential andsaid bridge input terminals;

said bridge output terminals being coupled to said control electrodepair of said controlled conduction device.

12. The combustion control circuit of claim 11, said controlledconductive device comprising an SCR.

13. The combustion control circuit of claim 11, said detection devicecomprising a photoelectric cell.

14. The combustion control circuit of claim 11, said driving signalmeans including a zener diode.

15. The combustion control circuit of claim 11, said flame detectiondevice having dark and light resistance conditions, said fixedresistances being chosen so that the bridge is unbalanced in response tothe absence of flame and oppositely unbalanced in response to thepresence of flame.

16. A combustion control circuit for controlling a fuel burnerinstallation including a burner motor and an ignition circuit of thetype including an ignition circuit gated device periodically renderedconductive by a gating signal to create a burner ignition condition,said combustion control circuit including:

motor start means and motor hold means coupled to the burner motor forsequentially initiating and then maintaining operation of the burnermotor;

a normally conductive controlled conduction device including electrodesforming an output electrode pair and a control electrode pair;

said output electrode pair being coupled to said motor start means forcontrolling the energization thereof;

a variable resistance flame detection device disposed in flame sensingrelation to the burner;

a resistance bridge having opposed input and output terminals and havingone leg including said flame detection device; and

driving signal means coupled between a source of operating potential andsaid bridge input terminals;

said bridge output terminals being coupled to said control electrodepair of said controlled conduction device for rendering said controlledconduction device nonconductive in response to detection of flame todisable said motor start means;

and an ignition disabling circuit coupled between said output electrodepair and the ignition circuit gated device for disabling the gateddevice in response to detection of flame.

17. A combustion control circuit for a fuel burner installation of thetype including a burner motor and an ignition circuit having an activeelement, said combustion control circuit comprising in combination:

a triac having agate electrode and having output electrodes connected incurrent supplying relation between a power source and both said motorand said ignition circuit;

a thermal switch connected in current supplying relation with the motor;I

a flame sensitive variable resistance device adjacent the burner;

a bridge circuit including said device in one leg;

means for driving said bridge with a regulated voltage;

a capacitor connected to said bridge for storing a potentialcorresponding to the balance condition of the bridge;

a controlled conduction device having a control electrode coupled tosaid capacitor for conduction in response to the absence of flame andfor nonconduction in response to the absence of flame;

a relay having pull-in winding means and hold winding means and having anormally open set of contacts coupled to the gate electrode of saidtriac;

means including a thermostat for energizing said hold winding means andsaid controlled conduction device in response to a demand for heat;

said pull-in winding means being connected for energization by saidcontrolling conduction device;

an ignition disabling means coupled between said controlled conductiondevice and the ignition circuit active element for disabling the activeelement in response to nonconduction of said controlled conductiondevice;

a first heater for said thermal switch connected in circuit with saidpull-in winding means; and

a second heater for said thermal switch connected in circuit with saidignition circuit.

18. The circuit of claim 17, said thermal switch being in series withsaid triac.

19. The circuit of claim 17, said controlled conduction devicecomprising an SCR.

20. The circuit of claim 17, said thermostat being in series with saidtriac.

21. The circuit of claim 17, said thermostat being connected in currentsupplying relation to said pull-in winding means and said hold windingmeans.

22. The circuit of claim 17, said disabling means comprising atransformer.

23. The circuit of claim 17, said disabling means comprising a relay.

24. The circuit of claim 17, said disabling means comprising a lamp anda light sensitive resistance.

25. A combustion control circuit for a fuel burner and for an ignitioncircuit including an ignition circuit SCR and including a gating circuitfor periodically applying a gating signal to the cathode and gateelectrodes of the ignition circuit SCR for producing an ignitioncondition, said combustion control circuit comprising:

control means for operating the burner in response to energization ofthe control means; a solid state controlled conduction device having acontrol electrode and having output electrodes coupled to said controlmeans for controlling the energization thereof;

a flame detection circuit coupled to the control electrode of said solidstate controlled conductive device for rendering said devicenonconductive in response to detection of burner flame; and

an ignition disabling circuit coupled between said solid statecontrolled conduction device and said ignition circuit;

said disabling circuit including a transformer having one windingconnected across the output electrodes of the solid state controlledconduction device and having another winding connected across thecathode and gate electrodes of the ignition circuit SCR to reverse biasthe ignition circuit SCR in response to nonconduction of said solidstate controlled conductive device.

A combustion control circuit for a fuel burner and for an ignitioncircuit including an ignition circuit SCR and including a gating circuitfor periodically applying a gating signal to the cathode and gateelectrodes of the ignition circuit SCR for producing an ignitioncondition, said combustion control circuit comprising;

control means for operating the burner in response to energization ofthe control means;

a solid state controlled conduction device having output electrodescoupled to said control means for controlling the energization thereof;

a flame detection circuit coupled to the control electrode of said solidstate controlled conduction device for rendering said devicenonconductive in response to detection of burner flame; and

an ignition disabling circuit between said solid state controlledconduction device and said ignition circuit;

said disabling circuit including a lamp connected across the outputelectrodes of the solid state controlled conduction device and a lightsensitive resistance connected across the cathode and gate electrodes ofthe ignition circuit SCR to prevent gating of the ignition circuit SCRin response to nonconduction of said solid state controlled conductiondevice.

i t a a: is

1. A combustion control circuit for controlling the operation of a fuelburner having a burner motor and an ignition device and comprising: atriac in circuit with both the burner motor and the ignition device; athermostat; a control circuit for placing the triac in a conductivecondition to energize both the burner motor and the Ignition device inresponse to operation of the thermostat; a flame detector coupled to thecontrol circuit for providing a flame indication in response tooperation of the burner; means coupled to said control circuit fordisabling the ignition device in response to a flame indication; athermal delay switch for disabling the burner motor in response to beingheated to a given extent; a first heater connected in said controlcircuit for heating said thermal delay switch during the time periodbeginning with operation of the thermostat and ending with production ofa flame indication; and a second heater connected in circuit with theignition device for heating said thermal delay switch during operationof the ignition device.
 2. The combustion control circuit of claim 1,said thermal delay switch being connected in series with said triac. 3.A combustion control circuit for a fuel burner used to heat a space andincluding a burner motor and an ignition device, said combustion controlcircuit comprising: a switching means coupled to both the burner motorand the ignition device and operable to energize the burner motor andignition device simultaneously; a flame detector disposed adjacent thefuel burner to provide a flame indication; a thermostatically controlledswitch in the heated space movable to an operating position in responseto a demand for heat; a first control means coupled to said switchingmeans and to said thermostatically controlled switch for operating saidswitching means in response to movement of said thermostaticallycontrolled switch to the operating position; a second control meanscoupled to said ignition device and to said flame detector fordiscontinuing operation of said ignition device in response to a flameindication; a thermal delay switch connected to the burner motor forpreventing operation of the burner motor in response to being heated toa predetermined extent; a first heater adjacent said thermal delayswitch and connected to said first and second control means forenergization during operation of said first control means prior tooperation of said second control means; and a second heater adjacentsaid thermal delay switch and connected to said ignition device forenergization during operation of said ignition device.
 4. The combustioncontrol circuit of claim 3, said switching means comprising a triac. 5.The combustion control circuit of claim 4, said thermal delay switchbeing connected in series with said triac.
 6. The combustion controlcircuit of claim 5, said first control means including relay meanshaving a winding means and having a contact means connected incontrolling relation to said triac, and an SCR having output electrodesand a gate electrode, said thermostatically controlled switch beingcoupled to said output electrodes, and said flame detector beingconnected to said gate electrode.
 7. The combustion control circuit ofclaim 6, said flame detector comprising a light sensitive variableresistor, a bridge circuit including said detector in one leg, means fordriving said bridge with a regulated driving signal, and a capacitorcoupled to the bridge output and to the gate electrode of the SCR. 8.The combustion control circuit of claim 5, said second control meanscomprising a coupling transformer connected between said flame detectorand said ignition device.
 9. The combustion control circuit of claim 5,said second control means comprising a relay connected between saidflame detector and said ignition device.
 10. The combustion controlcircuit of claim 5, said second control means comprising a lamp andlight sensitive resistor connected between said flame detector and saidignition device.
 11. A combustion control circuit for a fuel burnercomprising: control means for operating the burner in response toenergization of the control means; a controlled conduction deviceincluding electrodes forming an output elecTrode pair and a controlelectrode pair; said output electrode pair being coupled to said controlmeans for controlling the energization thereof; a variable resistanceflame detection device disposed in flame sensing relation to the burner;a resistance bridge having opposed input and output terminals and havingone leg including said flame detection device and having three legsincluding only fixed resistance; and driving signal means coupledbetween a source of operating potential and said bridge input terminals;said bridge output terminals being coupled to said control electrodepair of said controlled conduction device.
 12. The combustion controlcircuit of claim 11, said controlled conductive device comprising anSCR.
 13. The combustion control circuit of claim 11, said detectiondevice comprising a photoelectric cell.
 14. The combustion controlcircuit of claim 11, said driving signal means including a zener diode.15. The combustion control circuit of claim 11, said flame detectiondevice having dark and light resistance conditions, said fixedresistances being chosen so that the bridge is unbalanced in response tothe absence of flame and oppositely unbalanced in response to thepresence of flame.
 16. A combustion control circuit for controlling afuel burner installation including a burner motor and an ignitioncircuit of the type including an ignition circuit gated deviceperiodically rendered conductive by a gating signal to create a burnerignition condition, said combustion control circuit including: motorstart means and motor hold means coupled to the burner motor forsequentially initiating and then maintaining operation of the burnermotor; a normally conductive controlled conduction device includingelectrodes forming an output electrode pair and a control electrodepair; said output electrode pair being coupled to said motor start meansfor controlling the energization thereof; a variable resistance flamedetection device disposed in flame sensing relation to the burner; aresistance bridge having opposed input and output terminals and havingone leg including said flame detection device; and driving signal meanscoupled between a source of operating potential and said bridge inputterminals; said bridge output terminals being coupled to said controlelectrode pair of said controlled conduction device for rendering saidcontrolled conduction device nonconductive in response to detection offlame to disable said motor start means; and an ignition disablingcircuit coupled between said output electrode pair and the ignitioncircuit gated device for disabling the gated device in response todetection of flame.
 17. A combustion control circuit for a fuel burnerinstallation of the type including a burner motor and an ignitioncircuit having an active element, said combustion control circuitcomprising in combination: a triac having a gate electrode and havingoutput electrodes connected in current supplying relation between apower source and both said motor and said ignition circuit; a thermalswitch connected in current supplying relation with the motor; a flamesensitive variable resistance device adjacent the burner; a bridgecircuit including said device in one leg; means for driving said bridgewith a regulated voltage; a capacitor connected to said bridge forstoring a potential corresponding to the balance condition of thebridge; a controlled conduction device having a control electrodecoupled to said capacitor for conduction in response to the absence offlame and for nonconduction in response to the absence of flame; a relayhaving pull-in winding means and hold winding means and having anormally open set of contacts coupled to the gate electrode of saidtriac; means including a thermostat for energizing said hold windingmeans and said controlled conduction device in response to a demand forheat; said pull-in windIng means being connected for energization bysaid controlling conduction device; an ignition disabling means coupledbetween said controlled conduction device and the ignition circuitactive element for disabling the active element in response tononconduction of said controlled conduction device; a first heater forsaid thermal switch connected in circuit with said pull-in windingmeans; and a second heater for said thermal switch connected in circuitwith said ignition circuit.
 18. The circuit of claim 17, said thermalswitch being in series with said triac.
 19. The circuit of claim 17,said controlled conduction device comprising an SCR.
 20. The circuit ofclaim 17, said thermostat being in series with said triac.
 21. Thecircuit of claim 17, said thermostat being connected in currentsupplying relation to said pull-in winding means and said hold windingmeans.
 22. The circuit of claim 17, said disabling means comprising atransformer.
 23. The circuit of claim 17, said disabling meanscomprising a relay.
 24. The circuit of claim 17, said disabling meanscomprising a lamp and a light sensitive resistance.
 25. A combustioncontrol circuit for a fuel burner and for an ignition circuit includingan ignition circuit SCR and including a gating circuit for periodicallyapplying a gating signal to the cathode and gate electrodes of theignition circuit SCR for producing an ignition condition, saidcombustion control circuit comprising: control means for operating theburner in response to energization of the control means; a solid statecontrolled conduction device having a control electrode and havingoutput electrodes coupled to said control means for controlling theenergization thereof; a flame detection circuit coupled to the controlelectrode of said solid state controlled conductive device for renderingsaid device nonconductive in response to detection of burner flame; andan ignition disabling circuit coupled between said solid statecontrolled conduction device and said ignition circuit; said disablingcircuit including a transformer having one winding connected across theoutput electrodes of the solid state controlled conduction device andhaving another winding connected across the cathode and gate electrodesof the ignition circuit SCR to reverse bias the ignition circuit SCR inresponse to nonconduction of said solid state controlled conductivedevice. A combustion control circuit for a fuel burner and for anignition circuit including an ignition circuit SCR and including agating circuit for periodically applying a gating signal to the cathodeand gate electrodes of the ignition circuit SCR for producing anignition condition, said combustion control circuit comprising; controlmeans for operating the burner in response to energization of thecontrol means; a solid state controlled conduction device having outputelectrodes coupled to said control means for controlling theenergization thereof; a flame detection circuit coupled to the controlelectrode of said solid state controlled conduction device for renderingsaid device nonconductive in response to detection of burner flame; andan ignition disabling circuit between said solid state controlledconduction device and said ignition circuit; said disabling circuitincluding a lamp connected across the output electrodes of the solidstate controlled conduction device and a light sensitive resistanceconnected across the cathode and gate electrodes of the ignition circuitSCR to prevent gating of the ignition circuit SCR in response tononconduction of said solid state controlled conduction device.